Thrombopoietin stimulates myelodysplastic syndrome granulocyte-macrophage and erythroid progenitor proliferation

Treatment of myelodysplastic syndrome with thrombomimetic drugs

Myelodysplastic syndromes (MDS) are clinically, genetically, and molecularly heterogeneous neoplastic diseases characterized by ineffective hematopoiesis leading to peripheral cytopenias. The severity of cytopenias influences oucome and is considered in prognostic scoring systems; thrombocytopenia, although not the most frequently observed at disease onset, is estimated to affect 40%-80% of MDS patients. As thrombocytopenia in MDS is determined by premature marrow destruction and programmed cell death, the use of thrombomimetic agents has been proposed in order to stimulate megakaryocyte differentation and proliferation. After early attempts of therapy of thrombocytopenic MDS patients with thrombopoietin and interleukin (IL)-11, clinical trials testing the activity of romiplostim and eltrombopag have been recently designed and have demonstrated good tolerability and efficacy in re-inducing megakaryocytopoiesis and in lowering the number of hemorragic events. The number of MDS patients receiving such treatments is still limited and a broader evaluation of the long-term effects and safety of these agents is ongoing.

Hematopoietic Growth Factors in the Treatment of Acquired Bone Marrow Failure States

In severe aplastic anemia (SAA), the use of hematopoietic growth factors (HGFs) to support blood counts is of limited value, as predicted by in vitro studies and measurement of endogenous serum levels of hematopoietic growth factors (HGF), which are markedly elevated. Benefit is usually only seen in those with less severe disease who are unlikely to require HGFs in practice. HGFs administered alone play no role in the treatment of SAA. The main indication for using HGFs, most often granulocyte colony-stimulating factor (G-CSF), in SAA has been to determine whether they increase the response rate to immunosuppressive therapy (IST) and improve survival. While earlier neutrophil recovery occurs when G-CSF is administered with IST, studies to date show no significant advantage in hematologic response or overall survival. Conflicting results have been reported concerning whether G-CSF increases the known risk of myelodysplastic syndrome (MDS)/acute myeloid leukemia (AML) after IST; follow-up of at least 10 years is required, lacking in many clinical studies reported to date. In MDS, HGFs have been used to counteract the intramedullary apoptosis, which leads to ineffective hematopoiesis. In several uncontrolled and controlled studies, especially in low-risk MDS, high-dose erythropoietin (EPO) or its glycosylated derivative darbepoetin (DPO), alone or in combination with G-CSF, increased hemoglobin levels and diminished the need for red blood cell transfusions, in selected patients with prior transfusion frequency of less than 2 units per month and EPO levels below 500 IU/L. Quality-of-life measures were claimed to have improved, but the cost-effectiveness of this approach is debated, as is safety with regard to the risk of progression. G-CSF is used in supportive care of MDS to improve neutropenia during infectious complications, but to date there is no compelling evidence for a survival benefit or alteration of the course of the disease through the use of HGFs in MDS.

Leukemic cells and the cytokine patchwork

BACKGROUND

In leukemia, the clonal population is characterized by a hierarchical organization. Although the majority of the leukemic population is generated after post-determinic divisions, a subset of cells retain undifferentiated "blast" morphology. In addition, leukemic cells often have numerical or structural chromosomal abnormalities, aberrant gene expression patterns, and abnormal cell surface marker profiles. Despite these differences when compared to normal bone marrow and blood cells, leukemic cell survival and proliferation, just like that of normal progenitor cells, is influenced by hematopoietic growth factors. A major issue is whether differential regulation of normal and leukemic hematopoietic cells by cytokines can be exploited in antileukemic treatment or, in contrast, whether in vivo cytokine therapy may even be harmful to the patients.

PROCEDURE

Here we review the results of recent experimental and clinical observations that investigated the influence of cytokines on leukemic cell growth and differentiation in vitro and in vivo.

RESULTS

The majority of studies indicate that hematopoietic growth factors are involved in the regulation of proliferation and terminal differentiation of leukemic blast cells. Genetic aberrations involving cytokines or their receptors may contribute to leukemogenesis. Abundant interactions, cross-lineage stimulation, and aberrant response patterns seem to transform the complex cytokine network regulation of normal hematopoiesis into an even more interlaced "patchwork" that controls leukemic hematopoiesis.

CONCLUSIONS

Since hematopoietic growth factors are present in high serum concentrations in patients with acute leukemia and myelodysplastic syndromes, consequences of possible interactions should be kept in mind even when well-defined human recombinant factors in single application are to be involved in antileukemic protocols.

The myelodysplastic syndrome(s): a perspective and review highlighting current controversies

The myelodysplastic syndrome (MDS) includes a diverse group of clonal and potentially malignant bone marrow disorders characterized by ineffective and inadequate hematopoiesis. The presumed source of MDS is a genetically injured early marrow progenitor cell or pluripotential hematopoietic stem cell. The blood dyscrasias that fall under the broad diagnostic rubric of MDS appear to be quite heterogeneous, which has made it very difficult to construct a coherent, universally applicable MDS classification scheme. A recent re-classification proposal sponsored by the World Health Organization (WHO) has engendered considerable controversy. Although the precise incidence of MDS is uncertain, it has become clear that MDS is at least as common as acute myelogenous leukemia (AML). There is considerable overlap between these two conditions, and the former often segues into the latter; indeed, the distinction between AML and MDS can be murky, and some have argued that the current definitions are arbitrary. Despite the discovery of several tantalizing pathophysiological clues, the basic biology of MDS is incompletely understood. Treatment at present is generally frustrating and ineffective, and except for the small subset of patients who exhibit mild marrow dysfunction and low-risk cytogenetic lesions, the overall prognosis remains rather grim. In this narrative review, we highlight recent developments and controversies within the context of current knowledge about this mysterious and fascinating cluster of bone marrow failure states.

Comparative analysis of the in vitro proliferation and expansion of hematopoietic progenitors from patients with aplastic anemia and myelodysplasia

Aplastic anemia (AA) and myelodysplasia (MDS) show great similarities in their biology. To date, however, it is still unclear to what extent hematopoietic progenitor cells (HPCs) from AA and MDS share biological properties and what the functional differences are between them. In trying to address this issue, in the present study we have analyzed, in a comparative manner, the proliferation and expansion capacities of bone marrow (BM) progenitor cells from AA and MDS in response to recombinant cytokines. BM samples from normal subjects (NBM) and patients with AA and MDS were enriched for HPC by immunomagnetic-based negative selection. Selected cells were cultured in the absence (control) or in the presence of early-acting cytokines (Mix I), or early-, intermediate- and late-acting cytokines (Mix II). Proliferation and expansion were assessed periodically. In NBM and MDS cultures apoptosis was also determined. In NBM cultures, Mix I induced a nine-fold increase in total cell numbers and a 3.6-fold increase in colony-forming cell (CFC) numbers. In Mix II-supplemented cultures, total cells were increased 643-fold, and CFC 12.4-fold. In AA cultures, no proliferation or expansion were observed in Mix I-supplemented cultures, whereas only a four-fold increase in total cell numbers was observed in the presence of Mix II. In MDS cultures, a 12-fold increase in total cells and a 2.9-fold increase in CFC were observed in the presence of Mix I; on the other hand, Mix II induced a 224-fold increase in total cells and a 5.9-fold increase in CFC. Apoptosis was reduced in cytokine-supplemented cultures from NBM. In contrast, Mix II induced a significant increase in the rate of apoptosis in MDS cultures. Our results demonstrate that, as compared to their normal counterparts, AA and MDS progenitors are deficient in their proliferation and expansion potentials. Such a deficiency is clearly more pronounced in AA cells, which seem to be unable to respond to several cytokines. MDS progenitors, on the other hand, are capable to proliferate and expand in response to cytokines; however, their rate of apoptosis is increased by intermediate- and late-acting cytokines, so that the overall proliferation and expansion are significantly lower than those of normal progenitor cells.